earth supply?

[oohthatslive!!]

i have a TT system which has a RCD etc. At the service head there is a earthing block sticking out the side of the kind u find in pme earthing types. there is nothing connected to it and so if i connected an earth to it and did a Ze test and get a low reading, then i can presume that the link is across inside and that pme is available?

 

[bernardgreen]

NO

Because the incoming neutral may not be suitable.

 

[oohthatslive!!]

oh right. wot do u mean by not suitable?

 

[plugwash]

To offer PME safely certain precautions must be taken in the supply network. Only the DNO can know if those precautions are being taken in your case and so they should be contacted before putting in an earth-neutral connection.

 

[ricicle]

The DNO have to earth their neutral to ground at various intervals along it's way back to the transformer to minimise the risk of you being left with no earth should a break in the neutral occur.

 

[jimbob100_99]

Request an Earth Terminal from your local supplier they will tell you if you can have TNCS supply or if indeed there is one already there for you... If not RCDS all the way on TT

 

[wiggles]

If the suppliers neutral becomes damaged/disconnected before it enters the cutout, does this mean that the neutral within the house (and earth on TNS/PME) would be at the same potential as live?

 

[RF Lighting]

Yep, and so would the earth if it is a T-N-CS supply

 

[plugwash]

If the suppliers neutral becomes damaged/disconnected before it enters the cutout, does this mean that the neutral within the house (and earth on TNS/PME) would be at the same potential as live?

If the supply is single phase TT or TN-S or is single phase TN-C-S with no bonded services then yes. If its single phase TN-C-S with bonded services then some current will flow down those and the neutral won't rise all the way to live (exactly how far it rises depends on both the loading and the loop impedance down the bonded services). If its a multi phase system the voltage on the neutral will depend on the loading balance as well.

Still even if the main bonding stops it risiing quite as high as mains live a broken suppliers neutral isn't something you want your earth connected to!

The risk from broken neutrals can be mitigated by regularlly tying down the neutral in the main cable to earth and then using special cable types on the branches out to the individual custoemrs. Since only the DNO can know if they have done this or not you should not connect your earth to thier neutral without thier permission.

Some might interpret a connected earth terminal on the cutout and/or a PME sticker as permission but IMO its always best to check.

 

[wiggles]

Ouch! Presumably this is pretty rare (in the UK at least). I guess in most situations where the supply cable becomes damaged (being severed by a JCB etc) it’s more likely to create a short circuit and fail completely.

Have you ever encountered this situation? I can’t think of a much more dangerous scenario as I assume that all appliances/electrics would appear to be dead (as in not working) but would actually be live along with any bonded metalwork.

Is this why suppliers are moving away from TNS towards PME? In the case of PME, are there any guidelines as to how often the cable is earthed?

 

[plugwash]

PME is another name for TN-C-S in other words the system where broken neutrals can be really nasty. However as I said by using special cable types for the branch cables the risk can be reduced to very low levels. In the rare case that it still happens providing main bonding is done everything rises up together so the only way you can get a shock is between the earthing system and the general mass of the earth.

TN-S means the earth and neutral only connect at a single point (the transformer) and are seperate elsewhere hence if the neutral floats up the earth doesn't. The main downside of TN-S is cost, the extra earth wire costs money. Also in many cases the TN-S earth was/is carried through lead sheaths which are now degrading.

 

[wiggles]

Sorry plugwash, I had posted before reading your first message - i'm a slow typer.

I see what your saying. With TN-C-S if your have a broken neutral then you have no earth*

To be in the same situation with TN-S the broken neutral is only dangerous if the earth is also broken at the same time*

*other than what may be introduced if there are any pipes with MEB.

So in this scenario if you live in a new house with TN-C-S with plastic water pipes and no gas supply you are stuffed!

I had foolishly assumed that TN-C-S must be preferred if it’s replacing TN-S but I should have known it would boil down to cost.

You mention special cables used to supply the runs to houses to mitigate these risks, what's special about them?

 

[Lectrician]

If the terminal block is on the side of the head and accessible, if you do a loop test directly to this block and it gives you a suitable reading, I would say that it would be fine to use.

If the supply should not be used, the link would not be put across.

 

[oohthatslive!!]

yea thats wot i thought. anyway i did wot u suggested and did a loop test on the terminal block and still got a high reading (150 ohms) and so its TT. Still some very interesting thoughts on the whole thing